An electrophotographic photoreceptor have been widely applied to an electrophotographic apparatus such as a copying machine, a printer and the like. Semi-conductor laser ray may be generated with a compact and low-cost system, and it has been mainly used as a light source of the electrophotographic apparatus in recent years. Therefore, an organic photoconductive substance (OPC) which is sensitive to semi-conductor laser ray (about 800 nm), has been a matter of interest as a material for an electrophotographic photoreceptor in the art, and investigation about OPC has been made energetically.
OPC is the material in which a charge generating meterial having photoconductivity is dispersed into a binder consisting of an organic resin. By forming OPC in the form of layer on a conductive support, an electrophotographic photoreceptor is obtained. This type of electrophotographic photoreceptor is referred to as an organic photoreceptor. As the charge generating meterial, a functional colorant having photoconductivity such as a stable or metastable phthalocyanine compound is generally employed.
The phthalocyanine compound generally has wide spectrosensitivity over a long wavelength region, good charge generating efficiency, good fastness, high sensitivity, and high durability. Therefore, a metal free phthalocyanine or a metal phthalocyanine compound such as .chi.-form metal free phthalocyanine, titanylphthalocyanine, and vanadylphthalocyanine has been applied to the organic photoreceptor.
The metal free phthalocyanine does not contain metal, and it is safe, stable and easy to be prepared. Specifically, sensitivity of .chi.-form metal free phthalocyanine is better than that of .alpha.-form or .beta.-form metal free phthalocyanine, and the .chi.-form metal free phthalocyanine is practically used as a charge generating meterial.
The .chi.-form metal free phthalocyanine (hereinafter, referred to as "H2-Pc") is generally prepared by milling (grinding) and/or solvent treating the .alpha.-form or the .beta.-form H2-Pc to transform the polymorph thereof. However, the conventional process has required a long period of time for transforming the polymorph. In addition, the resulting .chi.-form H2-Pc is not satisfactory pure, and it contains untransformed H2-Pc in the amount that a peak thereof is found at an X-ray diffraction spectrum (hereinafter, referred to as "XRD spectrum").
U.S. Pat. No. 3,357,989 specification discloses a process for preparing .chi.-form H2-Pc in which the commercially available .alpha.-form or .beta.-form H2-Pc is dry milled for a long period. The process requires dry milling over not less than 48 hours in order to transform .alpha.-form or .chi.-form to .chi.-form, or dry milling of up to 144 hours in order to further increase the transformation rate.
Japanese Patent Kokoku Publication No. 14106/1969 discloses a process for preparing .chi.-form H2-Pc in which H2-Pc pigment is subjected to acid pasting, and it was then dry milled. However, the process does not steadily provide the .chi.-form polymorph, as shown in the following Comparative Example 1.
Japanese Patent Kokoku Publication No. 8102/1970 discloses a process for preparing .chi.-form H2-Pc in which a small amount of .chi.-form H2-Pc is added to the commercially available .alpha.-form H2-Pc to seed, and the resulting mixture is wet milled in a ketone solvent such as methyl ethyl ketone. However, the process often provides .beta.-form H2-Pc.
Japanese Patent Kokoku Publications No. 78872/1991, 85554/1993, and 37588/1995 disclose a process for preparing .chi.-form H2-Pc in which the amorphous-like .alpha.-form H2-Pc obtained by acid pasting the H2-Pc which is previously synthesized is dry milled, or the high-purified H2-Pc is dry milled.
For example, Japanese Patent Kokoku Publication No. 78872/1991 discloses a process for preparing high-pure .chi.-form H2-Pc in which phthalonitrile is reacted in the presence of a strong base (DBU) to form H2-Pc (.beta.-form), the resulting crude reaction product is made soluble to a solvent (for example, it is complexed to dipotassiumphthalocyanine diglyme), the resulting soluble substance is purified with a solvent, the resulting purified soluble substance is restored to obtain high-pure H2-Pc, and the high-pure H2-Pc is ball milled. However, the process is complicated in that the high-pure H2-Pc have to be prepared previously by the steps of making soluble to a solvent and purifying.
Japanese Patent Kokoku Publication No. 37588/1995 discloses a process for preparing .chi.-form H2-Pc in which the .alpha.-form H2-Pc which is obtained by acid pasting, is dry milled to obtain the mixture of .alpha.-form H2-Pc and .chi.-form H2-Pc, a ketone containing organic solvent is added to the mixture to obtain .chi.-form H2-Pc. The .chi.-form H2-Pc obtained by the process shows rather good photoconductivity, but is not sufficient (Comparative Example B).
Japanese Patent Kokoku Publication No. 85554/1993 discloses a process similar to that disclosed in Japanese Patent Kokoku Publication No. 78872/1991 in which, for example, the H2-Pc having the polymorph different from .chi.-form and a purity of not less than 95% is ground without a solvent (dry milled) in the form of powder at a temperature of 40 to 12.degree. C. by using a grinding machine (mill medium mixing type). The process aims at obtaining a high-pure substance, but the resulting .chi.-form H2-Pc often contains a minor amount of untransformed .alpha.-form H2-Pc (Comparative Example 3), and the characteristics thereof as a charge generating meterial are insufficient as shown in the following Comparative Example C.